Prism for repointing reflector antenna main beam

ABSTRACT

A microwave prism is used to repoint an operational Direct-to-Home (DTH) or Very Small Aperture Terminal (VSAT) reflector antenna as part of a ground terminal to receive (or transmit) signals from a different satellite or orbital position without physically moving the reflector or the feed horn antenna. The microwave prism operates by shifting the radiated fields from the horn antenna generally perpendicular to the focal axis of the parabolic reflector in order to cause the main beam of the reflector to scan in response. For an existing reflector antenna receiving signals from an incumbent satellite, a prism has been designed to be snapped into place over the feed horn and shift the fields laterally by a calibrated distance. The structure of the prism is designed to be positioned and oriented correctly without the use of skilled labor. This system allows a satellite service provider to repoint their subscribers to a new satellite by shipping a self-install kit of the prism that is pre-configured to have the correct orientation and position on the feed antenna to correctly re-point the beam at a different satellite once the prism is applied. One benefit of the system is that unskilled labor, i.e., the subscribers themselves, can be used to repoint a large number of subscriber antennas in a satellite network rather than requiring the cost of a truck roll and a technician to visit every site. The microwave prisms to implement this functionality can be constructed in different ways, with homogeneous slabs or blocks, Gradient-Index (GRIN), multi-layered dielectric, geometric or graded-index Fresnel-zone, metasurface, or metamaterial prisms. The geometric and electrical constraints of the design are determined by the incumbent and target satellites, and the ground terminal location.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/182,992, filed on Feb. 23, 2021, which claims the benefit of priorityof U.S. Provisional Application No. 62/981,367, filed on Feb. 25, 2020,the contents of both of which are relied upon and incorporated herein byreference in their entirety.

BACKGROUND

The following references are herein incorporated by reference: U.S. Pat.No. 6,075,497, Chen et. al., “Multiple-feed Electromagnetic SignalReceiving Apparatus,” filed Jun. 30, 1997, granted Jun. 13, 2000; U.S.Pat. No. 9,722,316, Haziza, Dedi David, Horn lens antenna, filed Jul. 7,2014, granted Aug. 1, 2017; U.S. Pat. No. 10,158,177, Cook, Scott,“Antenna horn with suspended dielectric tuning vane,” filed Mar. 11,2016, granted Dec. 18, 2018.

Satellite communications at microwave frequencies, whetherunidirectional or bidirectional, allow large quantities of data to bedistributed across large geographic regions, but require large antennasthat must be precisely pointed at the desired satellite in order tomaintain a high signal strength. The most common satellite antenna formicrowave (C, X, Ku, Ka, and higher bands) applications is ahorn-illuminated reflector, either a center-fed or offset-fed parabolic(or nearly parabolic) reflector, of which a wide variety of shapes,sizes, and operational frequencies are available.

Mobile platforms and ground terminals that are used to communicate withnon-geostationary orbit (NGSO) satellites will typically have motorizedtracking systems and electronics to maintain the connection while eitherthe ground terminal or satellite is in motion relative to the other.However, this hardware is very expensive. For a stationary groundlocation communicating with a GEO (Geostationary Earth Orbit) satellite,a fixed antenna that is pointed at the satellite once and then locked inplace is cost effective and simple. However, the disadvantage of thefixed reflector is that changing the satellite to which the antenna isconnected requires skilled or semi-skilled labor and tools toaccomplish. This reduces the ability of the subscriber to change theirservice provider or broadcaster, and also restricts the ability of theservice provider or broadcaster to change satellites or operators forcapacity, commercial, or other reasons. It is desirable that, while theterminal and antenna costs remain low for the mass market, there is aneasy way for the subscriber to repoint their own antenna from anoriginal incumbent satellite to a new satellite, without tools, tuning,or significant effort.

This present disclosure introduces a system and method by which amicrowave prism or lens can be used by an unskilled person on areflector antenna by snapping to or otherwise mating with the horn in acontrolled orientation to point the main beam of the reflector toconnect to a different satellite.

Referring to FIG. 1 , parabolic reflector antennas 101 for SATCOMpurposes have at a minimum of a piece of shaped metal or conductivematerial reflector 103 in the shape of a paraboloid, a horn antenna 109that serves to feed or illuminate the reflector 103, as well as supportstructure 105, 107 to mount the components in the correct relativepositions and fix the entire assembly to point rigidly at the satellite.A radome or cover 111 over the mouth of the horn protects it from wateror debris incursion. Some antennas will include additional shaped orparabolic subreflectors in the beam path to better control theillumination of the primary reflector, and/or modify the shape of theprimary reflector. The very low-cost antennas for Ku and Ka DTH mosttypically use an offset-fed reflector, which reduces the blockage causedby the feed horn. The feed horn will commonly be highly integrated withthe Low-Noise Block (LNB) Downconverter circuitry 113, and with amounting arm supporting the LNB and feed. A pole- or wall-mountingfixture is included on the back of the reflector that allows theorientation of the reflector and feed assembly to be adjusted and thenlocked into place by means of bolts or other fasteners.

New subscribers of a broadcast or bidirectional satellite service eitherpurchase the antenna 101 or are provided the antenna 101 as part of theservice. Although at times advertised as being able to be installed andpointed by the subscribers themselves, installation by a serviceprovider is almost universal.

The reflector 103, although presumably mounted to a solid structure andsecurely locked in place, can still become moved out of position bywind, snow, or other events. Correcting this problem requires a truckroll, which means sending a technician with training and tools tocorrectly re-point the antenna. Service visits represent a significantexpense to the service provider, even when the issue may take only a fewminutes to resolve.

Changing which satellite is connected when the antenna is not configuredwith multiple pre-pointed receivers requires both knowledge, tools, andskills. Currently, there are smartphone apps and websites that providedirection for how to point a satellite antenna, but the majority ofsubscribers would not be interested in doing so themselves. For thisreason, service providers are locked into particular orbital slots bytheir subscriber base—the more successful the broadcaster, the lessflexibility they have when trying to provide or modify the satellitefrom which they provide their service.

Microwave lenses and prisms constructed from dielectric, metamaterial,or metasurfaces are commonly used to control the radiation patterns ordirection of antennas. Microwave lenses use the same principles asoptical lenses, but use materials that have desirable properties forradio frequencies rather than optical wavelengths. Different featuresand methods have different benefits. Anti-reflective coatings arecommonly but not universally used, typically in microwave lensesimplemented as a quarter-wave plate or coating over the lens. Ananti-reflective coating serves to improve the impedance match of thesignal travelling from free space into the lens material, and again toimprove the impedance match of the signal exiting the lens. Due to thedifficulty of achieving the low dielectric constants needed for a goodanti-reflective coating, there are many methods of constructing suchlenses, including the use of foams, textured surfaces, and 3D printing.

Beam shifters are common devices in optics, composed of a polishedparallel-plate prism, which could also be described as a slab of glass.When rotated at various angles relative to an incident light beam, theexit point of the light from the prism is laterally shifted by adistance related to the incidence angle of the light and the thicknessof the prism. Such devices will include an optical anti-reflectivecoating, and are used as adjustment points in optical and laserworkbenches to align different parts of the system. A typical example isprovided by Thorlabs XYT/M-A Post-Mountable Tweaker Plate, 2.5 mm thick(Optical Beam Shifter), thorlabs.com.

SUMMARY

A reflector antenna repointing device for use with a reflector antenna.The reflector antenna repointing device has a microwave prism receivinginput fields and providing output fields. The device also has a mountingstructure configured to connect the prism to the reflector antenna. And,the device has adjustable alignment features at the mounting structureto set an adjustable position and adjustable orientation of themicrowave prism relative to the reflector antenna, wherein the alignmentfeatures define a lateral shift of the output fields relative to theinput fields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional parabolic reflector antenna.

FIGS. 2(a)-2(c) illustrate the principle of repointing the beam from areflector from an incumbent to a new target satellite using a prism.

FIG. 3 shows a parabolic reflector antenna equipped with a re-pointingprism snapped into place over the receiver.

FIG. 4 shows the components of the system.

FIGS. 5(a)-5(f) show multiple candidate prism implementations.

FIG. 6 shows the geometric considerations for the prism size andstructure.

FIG. 7 shows the orientation and angle that the terminal must pointrelative to the original satellite for a representative pair ofsatellites serving a geographical region.

FIG. 8(a) shows how the mounting of the prism allows the same prism tosupport multiple scan angle adjustments, with the alignment and prism ina first position E.

FIG. 8(b) is a side view of FIG. 8(a).

FIG. 8(c) is similar to FIG. 8(a) with the alignment and prism in asecond position A.

DETAILED DESCRIPTION

In describing the illustrative, non-limiting embodiments illustrated inthe drawings, specific terminology will be resorted to for the sake ofclarity. However, the disclosure is not intended to be limited to thespecific terms so selected, and it is to be understood that eachspecific term includes all technical equivalents that operate in similarmanner to accomplish a similar purpose. Several embodiments aredescribed for illustrative purposes, it being understood that thedescription and claims are not limited to the illustrated embodimentsand other embodiments not specifically shown in the drawings may also bewithin the scope of this disclosure.

Turning to the drawings, FIG. 3 shows a reflector antenna 200 having areflector repointing device 201. The reflector repointing device 201enables the main beam of a reflector antenna 200 to be steered by afixed and determined angle relative to the original angle of the antenna200 without the repointing device 201 installed. The reflectorrepointing device 201 is installed over the reflector's feed horn 109 ina position and orientation controlled by the device itself. The positionand orientation are set such that for a reflector antenna 200 within agiven geographic location already pointed at a specific satellite 213(FIG. 2(a)), the installation of the reflector repointing device willconvert the antenna to instead point at a separate, specific satellite215 without physically moving the reflector 103 or the feed horn 109.Separate devices 201 or different orientations of the same device 201can enable scanning within a range of +/− 10 degrees from the nominalangle. This is not a hard limit, but further scanning will result inmore significant performance degradation compared to the nominal casewithout the reflector repointing device installed. As a specificexample, an antenna 200 located within 50 miles of (for example)Washington DC and configured to receive signals from a satellite at 50°W could be converted to instead receive signals from a satellite at 45°W, without skilled installation or pointing calibration, by installingthe device 201 specifically designed for a 5 deg shift over the feed109.

For convenience, the following sections describe signals and fields asbeing transmitted from the antenna and reflector towards one or anothersatellite. The reciprocal behavior of transmission from the satelliteand reception by the antenna is not described, but is exactly analogousto the described case.

Referring to FIG. 2(a), the horn antenna 109 when in typical operationis sited such that the aperture of the horn is located at the focalpoint 203 of the parabolic reflector 103. In typical operation shown inFIG. 2(a), the system 101 is oriented such that the signals or antennafields 205 from the horn interact with the reflector 103 and aredirected towards the desired target satellite 213, forming a beam 207.Referring to FIG. 2(b), steering the system 101 to form a beam 211towards a different satellite 215 can be performed by reorienting theentire antenna 101, but can also be performed by shifting the feed horn109 away from the focal point 203 of the reflector 103. The antennafields 209 from the offset horn then interact with the reflector 103 toform a beam 211 directed at the alternate satellite 215. However, eitherre-orienting the entire antenna 101 or physically moving the horn 109both require skilled labor to perform, as well as support for themovement in the design of the reflector, neither can easily be performedwithout tools as a retrofit operation in most cases.

Turning to FIGS. 2(c), 3, the reflector repointing device 201 is addedto the horn 109 at the focal point 203 of the reflector 103 to laterallyshift the fields 205 that would ordinarily direct a beam 207 at theoriginal satellite 213 to instead produce a beam 211 at the alternatesatellite 215. Here a lateral shift indicates a direction perpendicularto the axis of symmetry of the parabolic reflector, which is also thedirection of the feed support arm. A lateral shift could be horizontal,vertical, or a combination of both, but should remain within the planealigned with the original feed aperture perpendicular to the reflectoraxis to ensure that the apparent phase center continues to lieapproximately on the focal plane of the reflector 103. The location ofthe feed aperture defines the starting point for comparison for allshifts. Laterally shifting and potentially correcting the angle of thefields effectively forces the reflector 103 to behave as though the horn109 was in a different location, thus generating a beam 211 in a new,desired direction. Referring now to FIG. 4 , the reflector repointingdevice 201 has a microwave prism or prism 401 with optionalanti-reflective coating features 403 at one or both surfaces (the topand bottom surfaces in the embodiment of FIG. 4 ) of the prism 401, amounting system or features 407 designed to connect and secure the prismto the host horn 109, and a radome or other cover 405 for protectionagainst the elements. All of the terms microwave prism, prism, microwavelens, and lens are intended to be applied to the device 401, and theterm prism is used in this disclosure to include microwave prisms.

The construction of the microwave prism has many similarities inpractice and principles with that of microwave lenses, including GRINlenses. A prism indicates a refractive device with a primary purpose ofbending or shifting a beam or cone, beam, or other distribution ofelectromagnetic energy, while a lens indicates a refractive device witha primary purpose of expanding or contracting a cone, beam or otherdistribution of electromagnetic energy. There is not a strict separationbetween these two concepts, as a prism can also be designed to focus,and a lens can also be designed to bend the energy. For this disclosure,prism is deemed more meaningful, as the primary purpose of the device401 is to bend and shift energy, rather than expand or contract,although some expansion and contraction may be included as well.

The prism 401 and mounting features 407 are specific to a particularmake or model of reflector antenna 101 and the accompanying horn 109,and will also be specific to particular satellites 213, 215. Themounting features 407 can be, for example, fasteners (such as bolts,nuts, screws), or adhesives.

As shown in FIG. 4 , in one embodiment, the microwave prism 401 has abody 402 with a first prism surface 402 a and a second prism surface 402b opposite the first surface 402 a. The prism body 402 has a top,bottom, and at least one side, and can have a cross-section of anysuitable shape, such as circular, square, or rectangular. The firstprism surface 402 a is at the top of the body 402 and the second prismsurface 402 b is at the bottom of the body 402. The first and secondprism surfaces 402 a, 402 b are planar. The anti-reflective coatingfeatures 403 can be a coating that is applied to the first and secondprism surfaces 402 a, 402 b. In one embodiment, the anti-reflectivecoating features have a top coating surface and a bottom coatingsurface, and a first coating feature 403 a has a bottom coating surfacethat contacts the top prism surface 402 a of the prism body 402, and asecond coating feature 403 b has a top coating surface that contacts thebottom prism surface 402 b of the prism body 402.

In the embodiment of FIG. 4 , the prism is a parallel-plate prism. Thefirst prism surface 402 a is parallel to the second prism surface 402 b.In addition, the flared sides of the horn 109 form a forward open mouth,and the forward edges of the flared sides form a forward perimeter thatis planar. The horn 109 also has a central longitudinal axis thatextends from the back of the horn to the front of the horn. The firstand second prism surfaces 402 a, 402 b are substantially parallel to theplanar mouth of the horn 109, and orthogonal to the longitudinal axis ofthe horn 109.

In one embodiment, the horn 109 and the reflector repointing device 201can instead be connected to a common support, such as a frame orhousing, and the mounting system or features 407 can connect thereflector repointing device 201 to the support and not to the horn 109.

The prism 401 functions by being positioned close to or at the apertureof the horn antenna 109 at a prescribed orientation, enforced by themounting features 407. In FIG. 5 , the prism 401, when appropriatelypositioned, produces a lateral offset 507 in the corresponding positionof the input and output fields 505 entering and exiting the device, withrespect to an original or undisturbed field position 506 of the antennafields from the horn 109 in the absence of a prism 401.

The antenna fields emanating from the feed horn are not as highlycollimated as those coming from a laser (due to the much longerwavelength of microwaves compared to the short wavelength of laserlight), and instead the fields are expanding in a spherical or conicalfashion between the feed horn 109 and reflector 103. Due to thisdifference between lasers and microwaves, so the reflector repointingdevice 201 can include corrections to allow for the cone-shapedemanation of energy coming from the horn, unlike the simple planar beamshifters for optical purposes. For example, the reflector repointingdevice 201 can have a non-planar surface (in particular, the bottomprism surface 402 b and/or the top prism surface 402 a) for nonzerooptical power to correct the field curvature and axial location of theeffective phase center of the output fields coming out of the repointingdevice 201.

The core operation of the prism 401 is to shift the fields laterallycompared to the location of the horn 109. That can be implemented in anysuitable manner, some examples of which are shown and described in theembodiments of FIGS. 5(a)-5(f) as various configurations for thereflector repointing device 201.

Referring to FIG. 5(a), the simplest option is for the microwave prism401 to be configured as an optical beam shifter or parallel plate prism401 a formed of a uniform high dielectric constant. The prism 401 isheld at a prescribed angle relative to the horn antenna 109, and morespecifically the first and/or second surfaces 402 a, 402 b are at anangle with respect to the plane of the open mouth of the horn 109 and atan angle with respect to the central longitudinal axis of the horn 109.This can optionally include anti-reflective layers 403 to support higherperformance. However, to create a large lateral shift or offset 507,this embodiment might require the prism to be very thick, have a highangle of incidence of the fields (which limits the transmissionefficiency), and a large dielectric constant ε. These factors combinedto make the prism option 401 a bulky and heavy.

In FIG. 5(b), another prism 401 b includes coupling and antireflectivelayers to more smoothly convert the direction of the signal throughoutthe structure with a series of one or more wedges 508. The prism 401 bhas a central body or plate 502 having a parallel shape, as with theprism 401 a, and one or more wedges 508 are connected (e.g., byadhesive) or integrally formed thereto extending outward from the topand/or bottom of the central plate 502. A first set of one or morewedges 508 a are arranged at the first side (the top) of the main body502, and a second set of one or more wedges 508 b are arranged at thesecond side (the bottom) of the main body 502.

The wedges 508 can have any suitable shape. However, in the embodimentshown, each wedge 508 is substantially triangular in shape with a firstplanar primary surface that faces the main body 502, a second planarprimary surface that faces away from the main body 502, and a smallsecondary surface. The bottom surface of the bottommost wedge of thefirst set of wedges 508 a, contacts the top surface 502 a of the mainbody 502, and the top surface of each wedge contacts the bottom surfaceof the adjacent wedge. The top surface of the topmost wedge of thesecond set of wedges 508 b, contacts the bottom surface 502 b of themain body 502, and the bottom surface of each wedge contacts the topsurface of the adjacent wedge. Each wedge has an acute angle formedbetween the first and second primary surfaces. In one embodiment, thefirst set of wedges 508 a have a combined angle that can be the same asthe offset angle θ of the bottom surface 402 b of the main body 502 withrespect to the plane of the mouth of the horn 109. And the second set ofwedges 508 b have a combined angle that can be the same as the offsetangle θ of the top surface 402 a of the main body 502 with respect tothe plane of the mouth of the horn 109. Accordingly, the bottom wedgesurface of the bottommost wedge 508 b of the lower wedge set issubstantially parallel to the plane of the horn mouth and to the topwedge surface of the topmost wedge 508 a of the top wedge set. Thus, theacute angle of the top set of wedges 508 a is aligned at one side of themain body 502 (i.e., the left side in the embodiment of FIG. 5(b)), andthe acute angle of the bottom set of wedges 508 b is aligned at theopposite side of the main body 502 (i.e., the right side). In thisconfiguration, the signal emerges substantially parallel to and offsetfrom the original signal axis 506, which can also be parallel to thecentral horn longitudinal axis.

In one embodiment, the multiple dielectric layers ε₁, ε₂, ε₃, ε₄, and ε₅for each wedge 508 are successively higher in dielectric constant thefurther from the central plate 502, with ε₁ the lowest and ε₅ thehighest. That is, the central plate 502 has the highest dielectricconstant, and each adjacent wedge 508 from the central plate 502 has asuccessively lower dielectric constant. This design allows increasedtransmission efficiency with the increased number of layers and muchsmaller field incidence angle at each layer but does little to minimizethe size and mass of the design. Thus, each wedge 508 refracts thesignal. And each wedge 508 of the bottom wedge set 508 b incrementallyincreases the angle of the signal with respect to the original axis 506.And each wedge 508 of the top wedge set 508 a reduces the angle of thesignal with respect to the original axis 506 until the signal issubstantially parallel to the original axis 506 or is otherwise at thedesired angle with respect to the original axis 506. An anti-reflectivecoating 403 can be placed at the top of the top wedge 506 a and at thebottom of the bottom wedge 506 b.

Turning to FIG. 5(c), a prism 401 c is shown with metamaterial andmetasurface technology, which also can include transmit array concepts.This embodiment reduces the mass of the prism 401 c by reducing thevolume of required material, but with a corresponding reduction in theoperational bandwidth and an increase in insertion loss. Rather than usebulk dielectric whose properties are defined by the dielectric constantand the shape, a metasurface prism has one or more layers ofmetamaterial or metasurface suspended in air by a supporting structure.For this implementation, a prism 401 c may be constructed from twolayers 531, 535 of a spatially varying metamaterial or metasurface thatchanges the direction of the fields from the horn 109 at two points byintroducing a phase gradient in the transmitted fields. The metamaterialor metasurface prism does not rely on refraction within a dielectricregion as does a conventional prism, and does not include the dielectricregions included in 401 a and 401 b. Accordingly, the bottommetamaterial or metasurface 531 refracts the signal away from theoriginal signal axis 506 so that the signal travels at an angle withrespect to the original signal axis 506. And the top metamaterial ormetasurface 535 refracts the signal back to being parallel to theoriginal signal axis 506. The double refraction offsets the signal fromthe original signal axis 506 and parallel thereto.

A gap or separation between the two layers 531, 535 is required to allowdistance for the fields to propagate and create the lateral offset. Thegreater the separation, the greater the lateral offset. The separationis maintained by a mechanical structure 533 internal to the prismstructure that maintains the space between the layers 531, 535 as an airgap. For example, the mechanical structure 533 can be a support or beamand one or both of the layers 531, 535 can be connected to the supportat different positions that maintains the desired air gap distancebetween them. This supporting structure internal to the prism isseparate in purpose and implementation from the structure 407 that holdsthe prism 401 to the feed 109, and can be implemented using supports,bolts, clips, or other physical features to maintain a fixed spacingbetween the two layers 531 and 535. The artificial dielectrics ormetasurface structures forming the layers 531 and 535 require periodicchanges to their structure across the surface of each layer 531 and 535to set up a phase gradient across the surfaces and therefore steer thebeam, which limits the usable bandwidth of the design. Metamaterial andmetasurface designs are often narrowband and lossy, but for someapplications may be sufficient. Transmission efficiency through bothlayers is a key metric for this style of implementation.

In FIG. 5(d), a corrugated prism 401 d is shown that reduces thethickness of the structure. That, in turn, reduces the weight, since thesupporting structure 407 and radome 405 can then be smaller as well. Thelarge height of the prisms shown in 401 a and 401 b is also reduced byintroducing shaped corrugations to the top and bottom surfaces of theprism 401 d. The corrugations have a sawtooth type shape. From the left,each tooth of the top surface has a straight leading rising edge that issubstantially parallel to the longitudinal axis of the horn, followed byan angled trailing falling edge.

The prism 401 d shows a collapsed version of the prism 401 a, but thesame approach could be applied to the multilayer 401 b. Collapsing thesize and shape of the prism by using a Fresnel-style corrugation in thetop and bottom surfaces 401 d can maintain largely the same beamsteering properties, but with a reduced height of the prism. This willproduce dispersive effects that limit the operational bandwidth, but islikely to have smaller dispersion than the metasurface/metamaterialapproach. An antireflective coating can be applied to the top and bottomcorrugated surfaces of 401 d, and will follow the shape of thecorrugations itself. The bottom surface refracts the signal to form anangle with respect to the original signal axis 506, and the top surfacerefracts the signal back to being parallel to (and offset from) theoriginal signal axis 506.

In FIG. 5(e), a graded-index or inhomogeneous prism 401 e, with fullcontrol over the internal dielectric constant ε(x,y,z) offerssignificant benefits of collapsing functionality into the smallestpossible contiguous package. The challenge in both smoothly varying(continuous) as well as stepped gradient designs is fabrication of theoften complex shapes and structures required to achieve the necessaryperformance.

With reference to FIG. 5(f), two half-prisms 401 f are shown. Since themass of the prism 401 is a major factor in the design of the repointingdevice 201, other actions to reduce the mass of the prism can be taken,including prisms where regions of dielectric in the interior of theprism are removed when unnecessary, effectively forming two half-prisms401 f that are separated from one another by a distance or gap of air.In some implementations, this gap might be implemented a hollow airregion might be constructed within an otherwise solid prism, reducingthe weight but not requiring a separate mounting or support structuresimilar to support 533. The half-prisms 401 f can have the shape of atriangle, with planar or curved surfaces. As shown, the upper half-prismcan have an inward-facing surface that is curved to be slightlyconcaved, and the lower half-prism can have an inward-facing surfacethat is curved to be slightly convex. The inward-facing surface of theupper half-prism faces and has a mating shape with the inward-facingsurface of the lower half-prism. Matching the profile of the innersurfaces of the prism to the propagation direction of the fields at eachangle allows the fields to continue straight without refraction at theinterface, as though the removed material were still there. Thismass-reduction approach is also useful when the loss tangent of theavailable dielectrics is high compared to air. The bottom surface of thelower half-prism 401 f is angled θ with respect to the plane of the hornmouth, and refracts the signal to be at an angle with respect to theoriginal signal axis 506. The top surface of the lower half-prism 401 ffurther refracts the signal. The bottom surface of the upper half-prism401 f refracts the signal back toward being parallel to the originalsignal axis 506, and the upper surface of the upper half-prism 401 ffurther refracts the signal to be parallel to and offset from theoriginal signal axis 506. The greater the distance between the upper andlower half-prisms 401 f, the greater the achievable lateral offset ofthe signal from the original signal axis 506.

Because the fields propagating through a dielectric region will notexpand by as much as if they were propagating only through air, theeffective phase center of the fields coming from the device 201 may nolonger match the reflector. Even though the lateral position may becorrect, the distance of the phase center of the feed distribution tothe reflector still needs to match the focal length of the reflector tomaintain aperture efficiency. The inclusion of nonzero optical gain(through curvature of the surface(s) or interior dielectric gradients)can be used to correct both the angular distribution of the fields aswell as the effective phase center.

The required size of the prism 401 is determined jointly by the degreeof lateral shift needed for the fields and the geometry of thereflector. A good prism should be small, lightweight, and compact inorder to minimize cost and simplify installation. However, the prism 401must be sized to intercept all of the power from the feed horn andredirect all of that energy to the reflector.

Referring to FIG. 6 , for reflectors with a small f/D (focal length 603to diameter 605) ratio, or equivalently a wide illumination cone angle609, a particular prism 611 implementation of a certain thickness mustbe large enough in the lateral directions to cover the originalradiation pattern cone 631 from the horn at the base of the prism, largeenough at the exit of the prism 611 to release energy across the entiresurface of the re-centered cone 633, and allow for enough internalthickness 615 and width 613 to reshape the energy sufficiently to followthe desired path 607. If a thicker 625 prism 621 implementation isrequired in order to provide the necessary lateral shift (in general,further shift requires a thicker prism to provide more room topropagate), then the prism (at least at the output) must be wider 623 aswell, thus making the volume and mass of the prism proportional(roughly, in general) to the cube of prism thickness. This leads to theimperative requirement to minimize the prism thickness while achievingother performance parameters, so as to also control the mass.

The f/D ratio also affects the amount of lateral shift required to steerthe reflector to a given angle. A reflector with a low f/D ratio, likemany common consumer DTH antennas, allows a small change in effectivefeed position 109 to produce a larger shift in beam scan angle.

Reflectors with high f/D would require a smaller prism to shift theaperture fields by a given distance since the cone angle is small, butwould require a larger physical shift to obtain the same scan angle ofthe main beam in degrees.

In one embodiment, the reflector repointing device 201 is retrofit to(and connected, such as by fastener mechanisms, adhesive or the like)existing horn antennas. Accordingly, it is configured to work withexisting horn antenna and parabolic reflector. The properties of theprism 401 are designed to suit the antenna system 101. However, in otherembodiments, the horn antenna and parabolic reflector can be designed towork with the device 201, which would involve mounting features 407ready for easy and precise installation of the device 201, a strongmounting arm 107 (FIG. 1 ) to support the additional weight of thedevice 201 without deflection, and a sufficiently large f/D ratio tooptimize the mass of the entire system 201. For a given pair ofsatellites, the incumbent satellite 213 and the new target satellite 215(FIG. 2 ), the correction angle that must be applied by the reflectorrepointing device 201 and the direction relative to the current antennapointing direction to which the offset should be applied is based on theangular distance between the two satellites and the location on theearth where the satellites are observed.

FIG. 7 shows the difference angles on the ground that must be applied.Since the relatively small-diameter reflectors 103 (typically between 40and 80 cm) used for DTH have fairly broad beams, the resolution of thecorrection is quite coarse. Based on the location of a targetinstallation of the reflector repointing device 201, a chart similar tothat in FIG. 7 is consulted and the necessary correction angle anddirection selected. The correction angle and direction are then appliedby implementation in an independent prism and mounting design for eachcombination, or by means of an adjustable fixture or mountingarrangement the scan angle and direction are set appropriately using asingle prism and fixture design for a broader geographic region. Theprism is effectively rotated about the central axis of the horn antenna109 by the correction angle in order to have the resulting beam bepointed at the target satellite based on the location of the system 200.The prism or setting selection can be performed before shipping thedevice to the end-user, or the end user can be provided the instructionsto use a particular numbered or labeled setting depending on theirlocation. i.e., the instructions might read, “For your postcode ABC,rotate the mounting clip to align the arrows with Position D beforeinstalling on your antenna”.

These instructions would apply to an implementation illustrated in FIG.8(a) and FIG. 8(b). A horn antenna 109 is shown mounted with its centerat point 811. The prism 401 (shown for illustrative purposes only, butcontained within the radome 405 of the device 201) shifts the fields ofthe horn to be re-centered to point 813 on leaving the prism. A set ofdetents and alignment markings are provided on the outer housing orradome material to be adjustably aligned either by the end user orbefore shipping. The alignment markings allow the user to adjust themounting structure to set an adjustable position and adjustableorientation of the microwave prism relative to the reflector antenna,and specifically relative to the horn feed 109. That is, the mountingstructure 407 fixedly attaches the device 201 to the antenna 101, but ata position and orientation that is defined by the adjustable alignmentmarkings. The alignment features define a plurality of positions andorientations of the microwave prism relative to the mounting structure,and can be set at the factory based on geographic location. The end usercan then determine (such as by using the map of FIG. 7 ) the propersetting based on the geographic location of the installation location.

Changing the orientation of the prism 401 relative to the central axisof the feed horn 109 changes the angle of the re-steered beam relativeto the original mounted antenna. For example, the prism can be rotatedabout the feed horn 109 central axis such that the resulting beam ispointed East or West of the original beam direction, as well asadjusting the elevation angle that the beam is pointed above the horizonto accurately point at the desired satellite 215. One or more supportmembers or fastening members can be provided to move the prism 401. Forinstance, a fastening member can movably connect the prism 401 to theradome 405 or to the mounting structure 407 so that the prism 401 canchange orientation relative to the feed horn 109.

FIG. 8(c) shows the device 401 aligned to position A, while FIG. 8(a)shows the device 401 aligned to position E. Here, it is noted that thesignal that is emerges from the horn is offset by the prism, as shown inFIGS. 5(a)-(f). The alignment 817 can be connected to the prism 401, sothat rotating the alignment 817 between the various positions, alsorotates the entire prism 401. When the prism and alignment 817 are atposition E (FIG. 8(a)), phase center 813 of the prism is offset from thephase center 811 of the horn. More specifically, the phase center 813 ofthe prism is at about 2 o'clock with respect to the phase center 811 ofthe horn. When the prism and alignment 817 are at position A (FIG.8(c)), the phase center 813 of the prism is at about 4 o'clock withrespect to the phase center 811 of the horn. Thus, the phase center 813of the prism rotates as the alignment rotates, which in turn moves thesignal that is output by the prism. In another embodiment, the differentpositions of the alignment can create different angles for the prism.

The prism 401 may take on any of the shapes or varying orientations withrespect to the feed horn as previously described in FIGS. 4-6 , and theshape and boundary of the radome is then selected to cover the prismappropriately.

After aligning the adjustable alignments 817, the device would fixedlyconnect to the horn in any suitable manner. For example, the mountingsystem 407 can include a snap connector 819 that snaps onto the mountingbar of the antenna, stabilized and oriented by the horn shroud 815. Onceinstalled, the device would now cause the reflector to point its mainbeam at the new desired satellite 215. The mounting features 407 fixedlyhold the device in the proper position to intercept all of the fieldsfrom the horn 109 that would otherwise reach the reflector 103, andshift the fields laterally to effect a change in scanning angle of thereflector antenna 101. No further motion or activity is then requiredduring proper operation of the antenna 101 and the device 201. If theantenna is desired to be repointed again at the original incumbentsatellite 213, then the device 201 can be removed by detaching themounting features 819.

It is noted that the drawings may illustrate, and the description andclaims may use geometric or relational terms, such as right, left,above, below, upper, lower, side, top, bottom, elongated, parallel,laterally, orthogonal, angle, rectangular, square, circular, round,axis. These terms are not intended to limit the disclosure and, ingeneral, are used for convenience to facilitate the description based onthe examples shown in the figures. In addition, the geometric orrelational terms may not be exact. For instance, signals and planes maynot be exactly perpendicular or parallel to one another but may still beconsidered to be perpendicular or parallel.

The foregoing description and drawings should be considered asillustrative only of the principles of the disclosure. The system may beconfigured in a variety of shapes and sizes and is not intended to belimited by the embodiment. Numerous applications of the system willreadily occur to those skilled in the art. Therefore, it is not desiredto limit the disclosure to the specific examples disclosed or the exactconstruction and operation shown and described. Rather, all suitablemodifications and equivalents may be resorted to, falling within thescope of the disclosure.

The invention claimed is:
 1. A prism assembly comprising: a prismarranged between a horn antenna and a reflector; a mounting structureconfigured to moveably connect the prism to the horn antenna; and two ormore alignment features configured to indicate a corresponding positionand orientation of the connected prism relative to the horn antenna,wherein the position and orientation of the prism indicated by each ofthe two or more alignment features has a corresponding effect on a beamfrom the horn antenna directed to the reflector, and the position andorientation of the prism is selectably adjusted to align with one of thetwo or more alignment features.
 2. The assembly of claim 1, wherein thehorn antenna comprises a horn feed that outputs the main beam.
 3. Theassembly of claim 1, wherein the prism redirects the main beam from afirst satellite to a second satellite.
 4. The assembly of claim 1,wherein the prism is a parallel-plate prism.
 5. The assembly of claim 1,wherein the prism includes one or more layers of metamaterial.
 6. Theassembly of claim 1, wherein the prism includes a graded-indexstructure.
 7. The assembly of claim 1, wherein the prism issubstantially enclosed by a radome.
 8. A system for repointing a mainbeam of a reflector antenna, comprising: a prism configured to receivethe main beam from a horn antenna and the prism configured to laterallyshift the main beam from the horn antenna in a direction that isperpendicular to an axis of symmetry of a reflector that is parabolic,and to direct the laterally shifted main beam to the reflector; and amounting structure configured to connect the prism to the horn antenna.9. The system of claim 8, wherein the mounting structure releasablyconnects to the horn antenna.
 10. The system of claim 8, wherein theprism is configured to repoint the main beam from a first satellite to asecond satellite.
 11. The system of claim 8, further comprising: one ormore alignment features configured to set a position of the prism and anorientation of the prism.
 12. The system of claim 11, wherein the one ormore alignment features are adjustable.
 13. A method for repointing amain beam of a reflector antenna, the method comprising: determining anoutput direction of the main beam from a horn antenna and determining anexit point of the main beam from the horn antenna; positioning a prismat the exit point to receive the main beam; and mounting the prism onthe horn antenna to shift the main beam, between the exit point from thehorn antenna to an output of the prism directed to a reflector,laterally in a direction that is substantially perpendicular to an axisof symmetry of the reflector.
 14. The method of claim 13, furthercomprising: one or more alignment features configured to set a positionof the prism and an orientation of the prism.
 15. The method of claim14, wherein the one or more alignment features are adjustable.
 16. Themethod of claim 13, wherein the prism is a parallel-plate prism.
 17. Themethod of claim 13, wherein the parallel-plate prism comprises aplurality of wedges.
 18. The method of claim 13, wherein the prism issubstantially enclosed by a radome.